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   <p data-fill-with="logo"><a class="logo" href="https://www.w3.org/"> <img alt="W3C" height="48" src="https://www.w3.org/StyleSheets/TR/2016/logos/W3C" width="72"> </a> </p>
   <h1 class="p-name no-ref" id="title">CSS Will Change Module Level 1</h1>
   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft, <time class="dt-updated" datetime="1970-01-01">1 January 1970</time></span></h2>
   <details>
    <summary>Specification Metadata</summary>
    <div data-fill-with="spec-metadata">
     <dl>
      <dt>This version:
      <dd><a class="u-url" href="https://drafts.csswg.org/css-will-change/">https://drafts.csswg.org/css-will-change/</a>
      <dt>Latest published version:
      <dd><a href="https://www.w3.org/TR/css-will-change/">https://www.w3.org/TR/css-will-change/</a>
      <dt>Previous Versions:
      <dd><a href="https://www.w3.org/TR/2015/CR-css-will-change-1-20151203/" rel="prev">https://www.w3.org/TR/2015/CR-css-will-change-1-20151203/</a>
      <dd><a href="https://www.w3.org/TR/2014/WD-css-will-change-1-20140429/" rel="prev">https://www.w3.org/TR/2014/WD-css-will-change-1-20140429/</a>
      <dt>Test Suite:
      <dd><a href="http://test.csswg.org/suites/css-will-change-1_dev/nightly-unstable/">http://test.csswg.org/suites/css-will-change-1_dev/nightly-unstable/</a>
      <dt class="editor">Editor:
      <dd class="editor p-author h-card vcard" data-editor-id="42199"><a class="p-name fn u-url url" href="http://xanthir.com/contact/">Tab Atkins Jr.</a> (<span class="p-org org">Google Inc.</span>)
      <dt>Suggest an Edit for this Spec:
      <dd><a href="https://github.com/w3c/csswg-drafts/blob/master/css-will-change-1/Overview.bs">GitHub Editor</a>
      <dt>Issue Tracking:
      <dd><a href="https://github.com/w3c/csswg-drafts/labels/css-will-change-1">GitHub Issues</a>
     </dl>
    </div>
   </details>
   <div data-fill-with="warning"></div>
   <p class="copyright" data-fill-with="copyright"><a href="https://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 1970 <a href="https://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="https://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="https://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="https://www.keio.ac.jp/">Keio</a>, <a href="https://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="https://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document">permissive document license</a> rules apply. </p>
   <hr title="Separator for header">
  </div>
  <div class="p-summary" data-fill-with="abstract">
   <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>
   <p>This document defines the <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change">will-change</a> CSS property, which allows an author to inform the UA ahead of time of what kinds of changes they are likely to make to an element.  This allows the UA to optimize how they handle the element ahead of time, performing potentially-expensive work preparing for an animation before the animation actually begins.</p>
    <a href="https://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, etc. 
  </div>
  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>
  <div data-fill-with="status">
   <p> This is a public copy of the editors’ draft.
	It is provided for discussion only and may change at any moment.
	Its publication here does not imply endorsement of its contents by W3C.
	Don’t cite this document other than as work in progress. </p>
   <p> <a href="https://github.com/w3c/csswg-drafts/issues">GitHub Issues</a> are preferred for discussion of this specification.
	When filing an issue, please put the text “css-will-change” in the title,
	preferably like this:
	“[css-will-change] <i data-lt>…summary of comment…</i>”.
	All issues and comments are <a href="https://lists.w3.org/Archives/Public/public-css-archive/">archived</a>,
	and there is also a <a href="https://lists.w3.org/Archives/Public/www-style/">historical archive</a>. </p>
   <p> This document was produced by the <a href="https://www.w3.org/Style/CSS/">CSS Working Group</a>. </p>
   <p> This document was produced by a group operating under
	the <a href="https://www.w3.org/Consortium/Patent-Policy/">W3C Patent Policy</a>.
	W3C maintains a <a href="https://www.w3.org/2004/01/pp-impl/32061/status" rel="disclosure">public list of any patent disclosures</a> made in connection with the deliverables of the group;
	that page also includes instructions for disclosing a patent.
	An individual who has actual knowledge of a patent which the individual believes contains <a href="https://www.w3.org/Consortium/Patent-Policy/#def-essential">Essential Claim(s)</a> must disclose the information in accordance with <a href="https://www.w3.org/Consortium/Patent-Policy/#sec-Disclosure">section 6 of the W3C Patent Policy</a>. </p>
   <p> This document is governed by the <a href="https://www.w3.org/2019/Process-20190301/" id="w3c_process_revision">1 March 2019 W3C Process Document</a>. </p>
   <p></p>
  </div>
  <div data-fill-with="at-risk"></div>
  <nav data-fill-with="table-of-contents" id="toc">
   <h2 class="no-num no-toc no-ref" id="contents">Table of Contents</h2>
   <ol class="toc" role="directory">
    <li>
     <a href="#intro"><span class="secno">1</span> <span class="content"> Introduction</span></a>
     <ol class="toc">
      <li><a href="#using"><span class="secno">1.1</span> <span class="content"> Using <span class="property">will-change</span> Well</span></a>
     </ol>
    <li><a href="#will-change"><span class="secno">2</span> <span class="content"> Hinting at Future Behavior: the <span class="property">will-change</span> property</span></a>
    <li><a href="#acks"><span class="secno">3</span> <span class="content">Acknowledgements</span></a>
    <li><a href="#changes"><span class="secno">4</span> <span class="content">Changes since the April 29 2014 Working Draft</span></a>
    <li>
     <a href="#conformance"><span class="secno"></span> <span class="content"> Conformance</span></a>
     <ol class="toc">
      <li><a href="#document-conventions"><span class="secno"></span> <span class="content"> Document conventions</span></a>
      <li><a href="#conform-classes"><span class="secno"></span> <span class="content"> Conformance classes</span></a>
      <li>
       <a href="#conform-responsible"><span class="secno"></span> <span class="content"> Requirements for Responsible Implementation of CSS</span></a>
       <ol class="toc">
        <li><a href="#conform-partial"><span class="secno"></span> <span class="content"> Partial Implementations</span></a>
        <li><a href="#conform-future-proofing"><span class="secno"></span> <span class="content"> Implementations of Unstable and Proprietary Features</span></a>
        <li><a href="#conform-testing"><span class="secno"></span> <span class="content"> Implementations of CR-level Features</span></a>
       </ol>
     </ol>
    <li>
     <a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
     <ol class="toc">
      <li><a href="#index-defined-here"><span class="secno"></span> <span class="content">Terms defined by this specification</span></a>
      <li><a href="#index-defined-elsewhere"><span class="secno"></span> <span class="content">Terms defined by reference</span></a>
     </ol>
    <li>
     <a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ol class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ol>
    <li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a>
   </ol>
  </nav>
  <main>
   <h2 class="heading settled" data-level="1" id="intro"><span class="secno">1. </span><span class="content"> Introduction</span><a class="self-link" href="#intro"></a></h2>
   <p>Modern CSS renderers perform a number of complex optimizations in order to render webpages quickly and efficiently.
	Unfortunately, employing these optimizations often has a non-trivial start-up cost,
	which can have a negative impact on the responsiveness of a page.</p>
   <div class="example" id="example-d1b18fd8">
    <a class="self-link" href="#example-d1b18fd8"></a> For example, when using CSS 3D Transforms to move an element around the screen,
		the element and its contents might be promoted to a “layer”,
		where they can render independently from the rest of the page and be composited in later.
		This isolates the rendering of the content so that the rest of the page doesn’t have to be rerendered
		if the element’s transform is the only thing that changes between frames,
		and often provides significant speed benefits. 
    <p>However, setting up the element in a fresh layer is a relatively expensive operation,
		which can delay the start of a <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-transforms-1/#propdef-transform" id="ref-for-propdef-transform">transform</a> animation by a noticeable fraction of a second.</p>
   </div>
   <p>The <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①">will-change</a> property defined in this specification allows an author to declare ahead-of-time what properties are likely to change in the future,
	so the UA can set up the appropriate optimizations some time before they’re needed.
	This way, when the actual change happens,
	the page updates in a snappy manner.</p>
   <h3 class="heading settled" data-level="1.1" id="using"><span class="secno">1.1. </span><span class="content"> Using <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②">will-change</a> Well</span><a class="self-link" href="#using"></a></h3>
   <p>The <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change③">will-change</a> property,
	like all performance hints,
	can be somewhat difficult to learn how to use “properly”,
	particularly since it has very little, if any, effect an author can directly detect.
	However, there are several simple “Dos and Don’ts”
	which hopefully will help develop a good intuition about how to use <span class="property" id="ref-for-propdef-will-change④">will-change</span> well.</p>
   <h4 class="no-num no-toc heading settled" id="dont-global"><span class="content"> Don’t Spam <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change⑤">will-change</a> Across Too Many Properties or Elements</span><a class="self-link" href="#dont-global"></a></h4>
   <p>A common initial response to seeing <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change⑥">will-change</a> is to assume that code like this is a good idea:</p>
<pre>* { will-change: transform, opacity /* , ... */; }</pre>
   <p>After all, this tells the browser to go ahead and optimize everything,
	which has to be good right?</p>
   <p>Wrong.  The browser <em>already</em> tries as hard as it can to optimize everything.
	Telling it to do so explicitly doesn’t help anything,
	and in fact has the capacity to do a lot of harm;
	some of the stronger optimizations that are likely to be tied to <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change⑦">will-change</a> end up using a lot of a machine’s resources,
	and when overused like this can cause the page to slow down or even crash.</p>
   <p>In addition, <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change⑧">will-change</a> does have <strong>some</strong> side-effects,
	and it’s very unlikely that pages actually want all those side-effects on every element.</p>
   <h4 class="no-num no-toc heading settled" id="css-sparingly"><span class="content"> Use <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change⑨">will-change</a> Sparingly In Stylesheets</span><a class="self-link" href="#css-sparingly"></a></h4>
   <p>Using <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①⓪">will-change</a> directly in a stylesheet
	implies that the targeted elements are always a few moments away from changing.
	This is <em>usually</em> not what you actually mean;
	instead, <span class="property" id="ref-for-propdef-will-change①①">will-change</span> should usually be flipped on and off via scripting
	before and after the change occurs (see <a href="#dont-waste">Don’t Waste Resources On Elements That Have Stopped Changing</a>).
	However, there are some common circumstances in which it is appropriate to use <span class="property" id="ref-for-propdef-will-change①②">will-change</span> directly in a stylesheet.</p>
   <div class="example" id="example-831888c0">
    <a class="self-link" href="#example-831888c0"></a> For example,
		specifying <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①③">will-change</a> for a small number of persistent UI elements in a page
		which should react snappily to the user
		is appropriate: 
<pre>body > .sidebar {
   will-change: transform;
   /* Will use 'transform' to slide it out
      when the user requests. */
}
</pre>
    <p>Because this is limited to a small number of elements,
		the fact that the optimization is rarely actually used
		doesn’t hurt very much.</p>
   </div>
   <div class="example" id="example-1ea11531">
    <a class="self-link" href="#example-1ea11531"></a> Sometimes an element really <em>does</em> change a property nearly constantly.
		Perhaps it responds to the user’s mouse movements,
		or just regularly takes some action that causes an animation.
		In this case, just declaring the <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①④">will-change</a> value in the stylesheet is fine,
		as it accurately describes that the element will regularly/constantly change,
		and so should be kept optimized. 
<pre>.cats-flying-around-the-screen {
  will-change: left, top;
}
</pre>
   </div>
   <h4 class="no-num no-toc heading settled" id="give-time"><span class="content"> Give <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①⑤">will-change</a> Sufficient Time To Work</span><a class="self-link" href="#give-time"></a></h4>
   <p>Another common bad pattern is to apply <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①⑥">will-change</a> to an element <em>immediately</em> before starting the animation or property change that it’s meant to help with.
	Unfortunately, most of those optimizations need time to be applied,
	and so they don’t have enough time to set-up when this is done,
	and the <span class="property" id="ref-for-propdef-will-change①⑦">will-change</span> has little to no effect.
	Instead, find some way to predict at least slightly ahead of time that something will change,
	and set <span class="property" id="ref-for-propdef-will-change①⑧">will-change</span> <em>then</em>.</p>
   <div class="example" id="example-8e9e08f2">
    <a class="self-link" href="#example-8e9e08f2"></a> For example,
		if an element is going to change when a user clicks on it,
		setting <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change①⑨">will-change</a> on hover will usually give at least 200 milliseconds
		for the optimizations to be set up,
		as human reaction time is relatively slow.
		This can be done either via scripting,
		or rather simply with a CSS rule: 
<pre>.element { transition: opacity .2s; opacity: 1; }
.element:hover { will-change: opacity; }
.element:active { opacity: .3; }
</pre>
    <p>However, a rule like that is useless if the effect is going to happen on hover.
		In cases like these, it is often still possible to find some way to predict the action before it occurs.
		For example, hovering an ancestor may give enough lead time:</p>
<pre>.element { transition: opacity .2s; opacity: 1; }
.container:hover > .element { will-change: opacity; }
.element:hover { opacity: .3; }
</pre>
   </div>
   <h4 class="no-num no-toc heading settled" id="dont-waste"><span class="content"> Don’t Waste Resources On Elements That Have Stopped Changing</span><a class="self-link" href="#dont-waste"></a></h4>
   <p>Because the optimizations browsers use for changing some properties are expensive,
	browsers remove them and revert to normal behavior as soon as they can in normal circumstances.
	However, <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②⓪">will-change</a> will generally override this behavior,
	maintaining the optimizations for much longer than the browser would otherwise do.</p>
   <p>As such, whenever you add <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②①">will-change</a> to an element,
	especially via scripting,
	don’t forget to <em>remove</em> it after the element is done changing,
	so the browser can recover whatever resources the optimizations are claiming.</p>
   <h2 class="heading settled" data-level="2" id="will-change"><span class="secno">2. </span><span class="content"> Hinting at Future Behavior: the <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②②">will-change</a> property</span><a class="self-link" href="#will-change"></a></h2>
   <table class="def propdef" data-link-for-hint="will-change">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn class="dfn-paneled css" data-dfn-type="property" data-export id="propdef-will-change">will-change</dfn>
     <tr class="value">
      <th><a href="https://drafts.csswg.org/css-values/#value-defs">Value:</a>
      <td class="prod">auto <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-one" id="ref-for-comb-one">|</a> <a class="production css" data-link-type="type" href="#typedef-animateable-feature" id="ref-for-typedef-animateable-feature">&lt;animateable-feature></a><a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#mult-comma" id="ref-for-mult-comma">#</a> 
     <tr>
      <th><a href="https://drafts.csswg.org/css-cascade/#initial-values">Initial:</a>
      <td>auto 
     <tr>
      <th>Applies to:
      <td><a href="https://drafts.csswg.org/css-pseudo/#generated-content" title="Includes ::before and ::after pseudo-elements.">all elements</a> 
     <tr>
      <th><a href="https://drafts.csswg.org/css-cascade/#inherited-property">Inherited:</a>
      <td>no 
     <tr>
      <th><a href="https://drafts.csswg.org/css-values/#percentages">Percentages:</a>
      <td>n/a 
     <tr>
      <th><a href="https://drafts.csswg.org/css-cascade/#computed">Computed value:</a>
      <td>specified value 
     <tr>
      <th>Canonical order:
      <td>per grammar 
     <tr>
      <th><a href="https://drafts.csswg.org/web-animations/#animation-type">Animation type:</a>
      <td>not animatable 
   </table>
<pre class="prod"><dfn class="dfn-paneled" data-dfn-type="type" data-export id="typedef-animateable-feature">&lt;animateable-feature></dfn> = scroll-position <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-one" id="ref-for-comb-one①">|</a> contents <a data-link-type="grammar" href="https://drafts.csswg.org/css-values-4/#comb-one" id="ref-for-comb-one②">|</a> <a class="production" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#identifier-value" id="ref-for-identifier-value">&lt;custom-ident></a></pre>
   <p>The <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②③">will-change</a> property provides a rendering hint to the user agent,
	stating what kinds of changes the author expects to perform on the element.
	This allows the user agent to perform ahead-of-time any optimizations necessary for rendering those changes smoothly,
	avoiding “jank” when the author does begin changing or animating that feature.</p>
   <div class="note" role="note"> Different browsers can use the information from <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②④">will-change</a> in different ways,
		and even a single browser might use it in different ways at different time.
		For example, a browser that promotes elements to their own “GPU layer”
		when they have <span class="css" id="ref-for-propdef-will-change②⑤">will-change: transform</span> specified
		might avoid doing that when there are <em>too many</em> elements declaring that,
		to avoid exhausting GPU memory. </div>
   <p>Values have the following meanings:</p>
   <dl>
    <dt><dfn class="dfn-paneled css" data-dfn-for="will-change" data-dfn-type="value" data-export id="valdef-will-change-auto">auto</dfn> 
    <dd> Expresses no particular intent;
			the user agent should apply whatever heuristics and optimizations it normally does. 
    <dt><dfn class="dfn-paneled css" data-dfn-for="will-change" data-dfn-type="value" data-export id="valdef-will-change-scroll-position">scroll-position</dfn> 
    <dd>
      Indicates that the author expects to animate or change the scroll position of the element in the near future. 
     <p class="example" id="example-c0594e8a"><a class="self-link" href="#example-c0594e8a"></a> For example, browsers often only render the content in the "scroll window" on a scrollable element,
				and some of the content past that window,
				balancing memory and time savings from the skipped rendering against making scrolling look nice.
				A browser might take this value as a signal to expand the range of content around the scroll window that is rendered,
				so that longer/faster scrolls can be done smoothly. </p>
    <dt><dfn class="dfn-paneled css" data-dfn-for="will-change" data-dfn-type="value" data-export id="valdef-will-change-contents">contents</dfn> 
    <dd>
      Indicates that the author expects to animate or change something about the element’s contents in the near future. 
     <div class="example" id="example-5dd10a2f">
      <a class="self-link" href="#example-5dd10a2f"></a> For example, browsers often “cache” rendering of elements over time,
				because most things don’t change very often,
				or only change their position.
				However, if an element <em>does</em> change its contents continually,
				producing and maintaining this cache is a waste of time.
				A browser might take this value as a signal to cache less aggressively on the element,
				or avoid caching at all and just continually re-render the element from scratch. 
      <p>This value is mostly intended to help browsers optimize JS-based animations of content,
				which change aspects of an element’s contents many times per second.
				This kind of optimization, when possible,
				is already done automatically by browsers when declarative animations are used.</p>
     </div>
     <p class="note" role="note"><span>Note:</span> This value more-or-less applies to the entire subtree of the element its declared on,
			as it indicates the browser should count on *any* of the descendants changing in some way.
			Using this on an element “high up” in your document might be very bad for your page’s performance;
			try to only use this on elements near the “bottom” of your document tree,
			containing as little of the document as possible.</p>
    <dt><dfn class="css" data-dfn-for="will-change" data-dfn-type="value" data-export id="valdef-will-change-custom-ident"><a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#identifier-value" id="ref-for-identifier-value①">&lt;custom-ident></a><a class="self-link" href="#valdef-will-change-custom-ident"></a></dfn> 
    <dd>
      If the <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#identifier-value" id="ref-for-identifier-value②">&lt;custom-ident></a> is an <a data-link-type="dfn" href="https://infra.spec.whatwg.org/#ascii-case-insensitive" id="ref-for-ascii-case-insensitive">ASCII case-insensitive</a> match for the name of a built-in CSS property,
			it indicates that the author expects to animate or change the property with the given name on the element in the near future.
			If the property given is a shorthand,
			it indicates the expectation for all the longhands the shorthand expands to. 
     <p class="example" id="example-03a1e696"><a class="self-link" href="#example-03a1e696"></a> For example, setting <a class="css" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②⑥">will-change: background;</a> is identical to setting <span class="css" id="ref-for-propdef-will-change②⑦">will-change: background-image, background-position, ...</span> for all the properties that <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-backgrounds-3/#propdef-background" id="ref-for-propdef-background">background</a> expands into. </p>
     <p>The <a class="production css" data-link-type="type" href="https://drafts.csswg.org/css-values-4/#identifier-value" id="ref-for-identifier-value③">&lt;custom-ident></a> production used here excludes the keywords <span class="css">will-change</span>, <span class="css">none</span>, <span class="css">all</span>, <a class="css" data-link-type="maybe" href="#valdef-will-change-auto" id="ref-for-valdef-will-change-auto">auto</a>, <a class="css" data-link-type="maybe" href="#valdef-will-change-scroll-position" id="ref-for-valdef-will-change-scroll-position">scroll-position</a>, and <a class="css" data-link-type="maybe" href="#valdef-will-change-contents" id="ref-for-valdef-will-change-contents">contents</a>,
			in addition to the keywords normally excluded from <span class="production" id="ref-for-identifier-value④">&lt;custom-ident></span>.</p>
     <p class="note" role="note"><span>Note:</span> Note that most properties will have no effect when specified,
			as the user agent doesn’t perform any special optimizations for changes in most properties.
			It is still <em>safe</em> to specify them, though;
			it’ll simply have no effect.</p>
     <p>Specifying a custom property must have no effect,
			which means that effects that happen through custom properties
			do not count for the rules below that are conditioned on any non-initial value of a property causing something.</p>
     <p class="note" role="note"><span>Note:</span> Specifying a value that’s not recognized as a property is fine;
			it simply has no effect.
			This allows you to safely specify <em>new</em> properties that exist in some user agents
			without negatively affecting down-level user agents
			that don’t know about that property.</p>
     <p class="example" id="example-7225dcbc"><a class="self-link" href="#example-7225dcbc"></a> For example, browsers often handle elements with <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-transforms-1/#propdef-transform" id="ref-for-propdef-transform①">transform</a> set to a non-initial value very differently from normal elements,
				perhaps rendering them to their own “GPU layer”
				or using other mechanisms to make it easier to quickly make the sort of transformations that <span class="property" id="ref-for-propdef-transform②">transform</span> can produce.
				A browser might take a value of <span class="property" id="ref-for-propdef-transform③">transform</span> as a signal
				that it should go ahead and promote the element to its own layer immediately,
				before the element starts to be transformed,
				to avoid any delay involved in rerendering the old and new layers. </p>
     <p>If any non-initial value of a property would create a stacking context on the element,
			specifying that property in <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②⑧">will-change</a> must create a stacking context on the element.</p>
     <p>If any non-initial value of a property would cause the element to generate a containing block for absolutely positioned elements,
			specifying that property in <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change②⑨">will-change</a> must cause the element to generate a containing block for absolutely positioned elements.</p>
     <p>If any non-initial value of a property would cause the element to generate a containing block for fixed positioned elements,
			specifying that property in <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change③⓪">will-change</a> must cause the element to generate a containing block for fixed positioned elements.</p>
     <p>If any non-initial value of a property would cause rendering differences on the element
			(such as using a different anti-aliasing strategy for text),
			the user agent should use that alternate rendering when the property is specified in <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change③①">will-change</a>,
			to avoid sudden rendering differences when the property is eventually changed.</p>
     <p class="example" id="example-e1300843"><a class="self-link" href="#example-e1300843"></a> For example, setting <a class="property" data-link-type="propdesc" href="https://drafts.csswg.org/css-color-4/#propdef-opacity" id="ref-for-propdef-opacity">opacity</a> to any value other than <span class="css">1</span> creates a stacking context on the element.
				Thus, setting <a class="css" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change③②">will-change: opacity</a> also creates a stacking context,
				even if <span class="property" id="ref-for-propdef-opacity①">opacity</span> is <em>currently</em> still equal to <span class="css">1</span>. </p>
   </dl>
   <p>The <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change③③">will-change</a> property has no <em>direct</em> effect on the element it is specified on,
	beyond the creation of stacking contexts and containing blocks as specified above.
	It is solely a rendering hint to the user agent,
	allowing it set up potentially-expensive optimizations for certain types of changes
	before the changes actually start occurring.</p>
   <h2 class="heading settled" data-level="3" id="acks"><span class="secno">3. </span><span class="content">Acknowledgements</span><a class="self-link" href="#acks"></a></h2>
   <p>Thanks to Benoit Girard for originally suggesting the <span class="css">will-animate</span> property,
and doing a lot of the initial design work.</p>
   <h2 class="heading settled" data-level="4" id="changes"><span class="secno">4. </span><span class="content">Changes since the April 29 2014 Working Draft</span><a class="self-link" href="#changes"></a></h2>
   <ul>
    <li data-md>
     <p>Added an explanatory section giving guidance on how to use <a class="property" data-link-type="propdesc" href="#propdef-will-change" id="ref-for-propdef-will-change③④">will-change</a> well.</p>
    <li data-md>
     <p>Specified the behavior of shorthands</p>
    <li data-md>
     <p>Change the animation type of the will-change property to not animatable</p>
   </ul>
  </main>
  <h2 class="no-ref no-num heading settled" id="conformance"><span class="content"> Conformance</span><a class="self-link" href="#conformance"></a></h2>
  <h3 class="heading settled" id="document-conventions"><span class="content"> Document conventions</span><a class="self-link" href="#document-conventions"></a></h3>
  <p>Conformance requirements are expressed with a combination of
	descriptive assertions and RFC 2119 terminology. The key words “MUST”,
	“MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”,
	“RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this
	document are to be interpreted as described in RFC 2119.
	However, for readability, these words do not appear in all uppercase
	letters in this specification. </p>
  <p>All of the text of this specification is normative except sections
	explicitly marked as non-normative, examples, and notes. <a data-link-type="biblio" href="#biblio-rfc2119">[RFC2119]</a> </p>
  <p>Examples in this specification are introduced with the words “for example”
	or are set apart from the normative text with <code>class="example"</code>,
	like this: </p>
  <div class="example" id="example-ae2b6bc0">
   <a class="self-link" href="#example-ae2b6bc0"></a> 
   <p>This is an example of an informative example. </p>
  </div>
  <p>Informative notes begin with the word “Note” and are set apart from the
	normative text with <code>class="note"</code>, like this: </p>
  <p class="note" role="note">Note, this is an informative note. </p>
  <p>Advisements are normative sections styled to evoke special attention and are
	set apart from other normative text with <code>&lt;strong class="advisement"></code>, like
	this: <strong class="advisement"> UAs MUST provide an accessible alternative. </strong> </p>
  <h3 class="heading settled" id="conform-classes"><span class="content"> Conformance classes</span><a class="self-link" href="#conform-classes"></a></h3>
  <p>Conformance to this specification
	is defined for three conformance classes: </p>
  <dl>
   <dt>style sheet 
   <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#style-sheet">CSS
			style sheet</a>. 
   <dt>renderer 
   <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that interprets the semantics of a style sheet and renders
			documents that use them. 
   <dt>authoring tool 
   <dd>A <a href="http://www.w3.org/TR/CSS2/conform.html#user-agent">UA</a> that writes a style sheet. 
  </dl>
  <p>A style sheet is conformant to this specification
	if all of its statements that use syntax defined in this module are valid
	according to the generic CSS grammar and the individual grammars of each
	feature defined in this module. </p>
  <p>A renderer is conformant to this specification
	if, in addition to interpreting the style sheet as defined by the
	appropriate specifications, it supports all the features defined
	by this specification by parsing them correctly
	and rendering the document accordingly. However, the inability of a
	UA to correctly render a document due to limitations of the device
	does not make the UA non-conformant. (For example, a UA is not
	required to render color on a monochrome monitor.) </p>
  <p>An authoring tool is conformant to this specification
	if it writes style sheets that are syntactically correct according to the
	generic CSS grammar and the individual grammars of each feature in
	this module, and meet all other conformance requirements of style sheets
	as described in this module. </p>
  <h3 class="heading settled" id="conform-responsible"><span class="content"> Requirements for Responsible Implementation of CSS</span><a class="self-link" href="#conform-responsible"></a></h3>
  <p>The following sections define several conformance requirements
		for implementing CSS responsibly,
		in a way that promotes interoperability in the present and future. </p>
  <h4 class="heading settled" id="conform-partial"><span class="content"> Partial Implementations</span><a class="self-link" href="#conform-partial"></a></h4>
  <p>So that authors can exploit the forward-compatible parsing rules to assign fallback values, <strong>CSS renderers <em>must</em> treat as invalid
		(and <a href="http://www.w3.org/TR/CSS2/conform.html#ignore">ignore as appropriate</a>)
		any at-rules, properties, property values, keywords, and other syntactic constructs
		for which they have no usable level of support</strong>.
		In particular, user agents <em>must not</em> selectively ignore
		unsupported property values and honor supported values in a single multi-value property declaration:
		if any value is considered invalid (as unsupported values must be),
		CSS requires that the entire declaration be ignored. </p>
  <h4 class="heading settled" id="conform-future-proofing"><span class="content"> Implementations of Unstable and Proprietary Features</span><a class="self-link" href="#conform-future-proofing"></a></h4>
  <p>To avoid clashes with future stable CSS features,
		the CSSWG recommends <a href="http://www.w3.org/TR/CSS/#future-proofing">following best practices</a> for the implementation of <a href="http://www.w3.org/TR/CSS/#unstable">unstable</a> features and <a href="http://www.w3.org/TR/CSS/#proprietary-extension">proprietary extensions</a> to CSS. </p>
  <h4 class="heading settled" id="conform-testing"><span class="content"> Implementations of CR-level Features</span><a class="self-link" href="#conform-testing"></a></h4>
  <p>Once a specification reaches the Candidate Recommendation stage,
		implementers should release an <a data-link-type="dfn" href="http://www.w3.org/TR/CSS/#vendor-prefix">unprefixed</a> implementation
		of any CR-level feature they can demonstrate
		to be correctly implemented according to spec,
		and should avoid exposing a prefixed variant of that feature. </p>
  <p>To establish and maintain the interoperability of CSS across
		implementations, the CSS Working Group requests that non-experimental
		CSS renderers submit an implementation report (and, if necessary, the
		testcases used for that implementation report) to the W3C before
		releasing an unprefixed implementation of any CSS features. Testcases
		submitted to W3C are subject to review and correction by the CSS
		Working Group. </p>
  <p>
   Further information on submitting testcases and implementation reports
		can be found from on the CSS Working Group’s website at <a href="http://www.w3.org/Style/CSS/Test/">http://www.w3.org/Style/CSS/Test/</a>.
		Questions should be directed to the <a href="http://lists.w3.org/Archives/Public/public-css-testsuite">public-css-testsuite@w3.org</a> mailing list. 
<script src="https://www.w3.org/scripts/TR/2016/fixup.js"></script>
  </p>
  <h2 class="no-num no-ref heading settled" id="index"><span class="content">Index</span><a class="self-link" href="#index"></a></h2>
  <h3 class="no-num no-ref heading settled" id="index-defined-here"><span class="content">Terms defined by this specification</span><a class="self-link" href="#index-defined-here"></a></h3>
  <ul class="index">
   <li><a href="#typedef-animateable-feature">&lt;animateable-feature></a><span>, in §2</span>
   <li><a href="#valdef-will-change-auto">auto</a><span>, in §2</span>
   <li><a href="#valdef-will-change-contents">contents</a><span>, in §2</span>
   <li><a href="#valdef-will-change-custom-ident">&lt;custom-ident></a><span>, in §2</span>
   <li><a href="#valdef-will-change-scroll-position">scroll-position</a><span>, in §2</span>
   <li><a href="#propdef-will-change">will-change</a><span>, in §2</span>
  </ul>
  <aside class="dfn-panel" data-for="term-for-propdef-background">
   <a href="https://drafts.csswg.org/css-backgrounds-3/#propdef-background">https://drafts.csswg.org/css-backgrounds-3/#propdef-background</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-propdef-background">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-propdef-opacity">
   <a href="https://drafts.csswg.org/css-color-4/#propdef-opacity">https://drafts.csswg.org/css-color-4/#propdef-opacity</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-propdef-opacity">2. 
Hinting at Future Behavior: the will-change property</a> <a href="#ref-for-propdef-opacity①">(2)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-propdef-transform">
   <a href="https://drafts.csswg.org/css-transforms-1/#propdef-transform">https://drafts.csswg.org/css-transforms-1/#propdef-transform</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-propdef-transform">1. 
Introduction</a>
    <li><a href="#ref-for-propdef-transform①">2. 
Hinting at Future Behavior: the will-change property</a> <a href="#ref-for-propdef-transform②">(2)</a> <a href="#ref-for-propdef-transform③">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-mult-comma">
   <a href="https://drafts.csswg.org/css-values-4/#mult-comma">https://drafts.csswg.org/css-values-4/#mult-comma</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-mult-comma">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-identifier-value">
   <a href="https://drafts.csswg.org/css-values-4/#identifier-value">https://drafts.csswg.org/css-values-4/#identifier-value</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-identifier-value">2. 
Hinting at Future Behavior: the will-change property</a> <a href="#ref-for-identifier-value①">(2)</a> <a href="#ref-for-identifier-value②">(3)</a> <a href="#ref-for-identifier-value③">(4)</a> <a href="#ref-for-identifier-value④">(5)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-comb-one">
   <a href="https://drafts.csswg.org/css-values-4/#comb-one">https://drafts.csswg.org/css-values-4/#comb-one</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-comb-one">2. 
Hinting at Future Behavior: the will-change property</a> <a href="#ref-for-comb-one①">(2)</a> <a href="#ref-for-comb-one②">(3)</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="term-for-ascii-case-insensitive">
   <a href="https://infra.spec.whatwg.org/#ascii-case-insensitive">https://infra.spec.whatwg.org/#ascii-case-insensitive</a><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-ascii-case-insensitive">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
  <h3 class="no-num no-ref heading settled" id="index-defined-elsewhere"><span class="content">Terms defined by reference</span><a class="self-link" href="#index-defined-elsewhere"></a></h3>
  <ul class="index">
   <li>
    <a data-link-type="biblio">[css-backgrounds-3]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-propdef-background" style="color:initial">background</span>
    </ul>
   <li>
    <a data-link-type="biblio">[css-color-4]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-propdef-opacity" style="color:initial">opacity</span>
    </ul>
   <li>
    <a data-link-type="biblio">[css-transforms-1]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-propdef-transform" style="color:initial">transform</span>
    </ul>
   <li>
    <a data-link-type="biblio">[css-values-4]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-mult-comma" style="color:initial">#</span>
     <li><span class="dfn-paneled" id="term-for-identifier-value" style="color:initial">&lt;custom-ident></span>
     <li><span class="dfn-paneled" id="term-for-comb-one" style="color:initial">|</span>
    </ul>
   <li>
    <a data-link-type="biblio">[INFRA]</a> defines the following terms:
    <ul>
     <li><span class="dfn-paneled" id="term-for-ascii-case-insensitive" style="color:initial">ascii case-insensitive</span>
    </ul>
  </ul>
  <h2 class="no-num no-ref heading settled" id="references"><span class="content">References</span><a class="self-link" href="#references"></a></h2>
  <h3 class="no-num no-ref heading settled" id="normative"><span class="content">Normative References</span><a class="self-link" href="#normative"></a></h3>
  <dl>
   <dt id="biblio-css-values-4">[CSS-VALUES-4]
   <dd>Tab Atkins Jr.; Elika Etemad. <a href="https://www.w3.org/TR/css-values-4/">CSS Values and Units Module Level 4</a>. 31 January 2019. WD. URL: <a href="https://www.w3.org/TR/css-values-4/">https://www.w3.org/TR/css-values-4/</a>
   <dt id="biblio-infra">[INFRA]
   <dd>Anne van Kesteren; Domenic Denicola. <a href="https://infra.spec.whatwg.org/">Infra Standard</a>. Living Standard. URL: <a href="https://infra.spec.whatwg.org/">https://infra.spec.whatwg.org/</a>
   <dt id="biblio-rfc2119">[RFC2119]
   <dd>S. Bradner. <a href="https://tools.ietf.org/html/rfc2119">Key words for use in RFCs to Indicate Requirement Levels</a>. March 1997. Best Current Practice. URL: <a href="https://tools.ietf.org/html/rfc2119">https://tools.ietf.org/html/rfc2119</a>
  </dl>
  <h3 class="no-num no-ref heading settled" id="informative"><span class="content">Informative References</span><a class="self-link" href="#informative"></a></h3>
  <dl>
   <dt id="biblio-css-backgrounds-3">[CSS-BACKGROUNDS-3]
   <dd>Bert Bos; Elika Etemad; Brad Kemper. <a href="https://www.w3.org/TR/css-backgrounds-3/">CSS Backgrounds and Borders Module Level 3</a>. 17 October 2017. CR. URL: <a href="https://www.w3.org/TR/css-backgrounds-3/">https://www.w3.org/TR/css-backgrounds-3/</a>
   <dt id="biblio-css-color-4">[CSS-COLOR-4]
   <dd>Tab Atkins Jr.; Chris Lilley. <a href="https://www.w3.org/TR/css-color-4/">CSS Color Module Level 4</a>. 5 March 2019. WD. URL: <a href="https://www.w3.org/TR/css-color-4/">https://www.w3.org/TR/css-color-4/</a>
   <dt id="biblio-css-transforms-1">[CSS-TRANSFORMS-1]
   <dd>Simon Fraser; et al. <a href="https://www.w3.org/TR/css-transforms-1/">CSS Transforms Module Level 1</a>. 14 February 2019. CR. URL: <a href="https://www.w3.org/TR/css-transforms-1/">https://www.w3.org/TR/css-transforms-1/</a>
  </dl>
  <h2 class="no-num no-ref heading settled" id="property-index"><span class="content">Property Index</span><a class="self-link" href="#property-index"></a></h2>
  <div class="big-element-wrapper">
   <table class="index">
    <thead>
     <tr>
      <th scope="col">Name
      <th scope="col">Value
      <th scope="col">Initial
      <th scope="col">Applies to
      <th scope="col">Inh.
      <th scope="col">%ages
      <th scope="col">Anim­ation type
      <th scope="col">Canonical order
      <th scope="col">Com­puted value
    <tbody>
     <tr>
      <th scope="row"><a class="css" data-link-type="property" href="#propdef-will-change" id="ref-for-propdef-will-change③⑤">will-change</a>
      <td>auto | &lt;animateable-feature>#
      <td>auto
      <td>all elements
      <td>no
      <td>n/a
      <td>not animatable
      <td>per grammar
      <td>specified value
   </table>
  </div>
  <aside class="dfn-panel" data-for="propdef-will-change">
   <b><a href="#propdef-will-change">#propdef-will-change</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-propdef-will-change①">1. 
Introduction</a>
    <li><a href="#ref-for-propdef-will-change②">1.1. 
Using will-change Well</a> <a href="#ref-for-propdef-will-change③">(2)</a> <a href="#ref-for-propdef-will-change④">(3)</a>
    <li><a href="#ref-for-propdef-will-change⑤"> Don’t Spam will-change Across Too Many Properties or Elements</a> <a href="#ref-for-propdef-will-change⑥">(2)</a> <a href="#ref-for-propdef-will-change⑦">(3)</a> <a href="#ref-for-propdef-will-change⑧">(4)</a>
    <li><a href="#ref-for-propdef-will-change⑨"> Use will-change Sparingly In Stylesheets</a> <a href="#ref-for-propdef-will-change①⓪">(2)</a> <a href="#ref-for-propdef-will-change①①">(3)</a> <a href="#ref-for-propdef-will-change①②">(4)</a> <a href="#ref-for-propdef-will-change①③">(5)</a> <a href="#ref-for-propdef-will-change①④">(6)</a>
    <li><a href="#ref-for-propdef-will-change①⑤"> Give will-change Sufficient Time To Work</a> <a href="#ref-for-propdef-will-change①⑥">(2)</a> <a href="#ref-for-propdef-will-change①⑦">(3)</a> <a href="#ref-for-propdef-will-change①⑧">(4)</a> <a href="#ref-for-propdef-will-change①⑨">(5)</a>
    <li><a href="#ref-for-propdef-will-change②⓪"> Don’t Waste Resources On Elements That Have Stopped Changing</a> <a href="#ref-for-propdef-will-change②①">(2)</a>
    <li><a href="#ref-for-propdef-will-change②②">2. 
Hinting at Future Behavior: the will-change property</a> <a href="#ref-for-propdef-will-change②③">(2)</a> <a href="#ref-for-propdef-will-change②④">(3)</a> <a href="#ref-for-propdef-will-change②⑤">(4)</a> <a href="#ref-for-propdef-will-change②⑥">(5)</a> <a href="#ref-for-propdef-will-change②⑦">(6)</a> <a href="#ref-for-propdef-will-change②⑧">(7)</a> <a href="#ref-for-propdef-will-change②⑨">(8)</a> <a href="#ref-for-propdef-will-change③⓪">(9)</a> <a href="#ref-for-propdef-will-change③①">(10)</a> <a href="#ref-for-propdef-will-change③②">(11)</a> <a href="#ref-for-propdef-will-change③③">(12)</a>
    <li><a href="#ref-for-propdef-will-change③④">4. Changes since the April 29 2014 Working Draft</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="typedef-animateable-feature">
   <b><a href="#typedef-animateable-feature">#typedef-animateable-feature</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-typedef-animateable-feature">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="valdef-will-change-auto">
   <b><a href="#valdef-will-change-auto">#valdef-will-change-auto</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-will-change-auto">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="valdef-will-change-scroll-position">
   <b><a href="#valdef-will-change-scroll-position">#valdef-will-change-scroll-position</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-will-change-scroll-position">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
  <aside class="dfn-panel" data-for="valdef-will-change-contents">
   <b><a href="#valdef-will-change-contents">#valdef-will-change-contents</a></b><b>Referenced in:</b>
   <ul>
    <li><a href="#ref-for-valdef-will-change-contents">2. 
Hinting at Future Behavior: the will-change property</a>
   </ul>
  </aside>
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